Intelligent Transportation Systems (ITS) have been proposed as traffic systems in which information communications technology is used to exchange information about people, roads, and vehicles in order to achieve prevention of traffic accidents, relief of traffic congestion, and so on. Examples of the ITS include a car navigation system and a road-traffic-information system (e.g., a VICS (Vehicle Information and Communication System)), an automatic toll payment system (e.g., ETC (Electronic Toll Collection system)), and a bus location system.
In such ITS technologies, vehicle-to-vehicle communication, in which multiple vehicles communicate with each other, and road-to-vehicle communication, in which communication is performed between base stations provided along roads and vehicles, are performed to transmit/receive information.
Japanese Laid-Open Patent Publication No. 2006-31573 discusses a drive assist system that is configured so that vehicles obtain road information by utilizing the road-to-vehicle communication and instructions about vehicle travel speed conditions are issued so that the vehicles do not need to stop at red signals at intersections.
In the above Publication document, it is proposed that a recommended speed for enabling the vehicles to pass through an intersection is calculated at a point that is away from the intersection by a certain distance and speed control is performed so that the vehicles do not need to stop at the red signal at the intersection.
In the above Publication document, however, the proposed technology is based on the premise that all vehicles travel straight at the intersections and does not consider a speed reduction due to, for example, right and left turns of the vehicles traveling ahead at the intersection.
Japanese Laid-Open Patent Publication No. 2006-251836 discusses a system that is configured to recalculate a recommended speed of a vehicle at regular time intervals or regular travel distances, considering, for example, the states of vehicles traveling ahead.